1. Field of the Invention
The present invention relates to through tubing perforation guns used to support explosive charges in a borehole to form perforations through which water, petroleum or minerals are produced.
2. Background Information
This invention is an improvement to prior art phased, through tubing, perforating systems in that it allows for widely varied phasing (i.e., orientation of multiple directional charges at various angles) while allowing for retrieval of the carrier. Prior art phased capsule perforating systems may be generally classified into three categories: (1) the phased frangible base strip (U.S. Pat. No. 4,951,744); (2) the retrievable base strip with frangible retaining means (U.S. Pat. No. 5,095,999); and (3) the phased expendable link (U.S. Pat. No. 5,241,891).
The disadvantages of the first category (illustrated in FIG. 1 of the drawings) is that the shattered pieces of the base strip are not retrieved from the well leaving a substantial amount of debris. As a result, one cannot determine if all the charges detonated properly. Also, since the base strip shatters after firing the gun, the strip must be brittle and thereby could break when it is not desirable (e.g. upon conveying in the well).
The disadvantage of the second category (illustrated in FIG. 2 of the drawings) is that the base strip is composed of a heavy gauge steel bar that limits possible phasing (normally +45 degrees, -45 degrees) and that distorts (when the shaped charges are fired) to make retrieval difficult. Also, since only a relatively weak breakable clip retains the capsule charge to the base strip, it may break when it is undesirable (e.g. upon conveying into the well). The advantage of this system is that it permits some simple phasing (two rows at .+-.45 degrees typically), and the strip is rugged and retrievable.
The disadvantages of the third category (illustrated in FIG. 3 of the drawings) are that more debris is left in the well and that the system is weak (the pins and links often break when they hit obstructions in the tubing), resulting in use only for simple perforating operations. The main advantage of the third category is that very flexible phasing is possible. This high degree of phasing of the capsules is significant to well productivity in many formation types.
The above-mentioned co-pending application discloses a perforating gun carrier with a slotted configuration and interior dimensions to enable capsule orientation at selected phases between 0 and 360 degrees. The carrier has a frangible seam that fractures upon detonation to form two retrievable strips, each supported by the conveyance sub for retrieval. The seam is a narrow bridge, formed by slotting the carrier partially, with a cross-sectional area that shatters upon detonation of the shaped charges. The remaining cross-sectional area and strength of each strip is sufficient to assure retrieval after detonation. The strips are preferably nonplanar, arcuate or a segment of a circle in cross section. When the capsule charges are arrayed around many phases, by attaching both front and rear portions of the capsule charges to the nonfrangible regions of the carrier, detonating cords are used for detonation.